September 19, 2013

Just How Many Scents Can We Smell?

Of all the different odors and smells floating around the globe, scientists say humans can only detect ten basic categories of scents. Using a computerized model, sensory scientists now say they've developed a way to systematically categorize these smells.

Using a preexisting set of odors and descriptions, the scientists ran them through the computerized model, categorized them, and have now concluded all the world’s smells largely fit into ten groups. It can be quite difficult to accurately categorize and describe a sense, and as such, there have been some who question the validity of this work.

Researchers have tried to tackle the science of the olfaction before, resulting in the development of an electronic nose which can chemically detect which sense is being presented. This new work, carried out by scientists from Bates College, the University of Pittsburgh and Oak Ridge National Laboratory, has now been published in the open-access journal PLOS ONE.

After looking at smells through a mathematical model, the scientists say humans generally describe smells as either fragrant, woody/resinous, fruity (non-citrus), chemical, minty/peppermint, sweet, popcorn, and lemon. Perhaps some good news from this study: The scientists say there are only two categories which cause a human’s stomach to turn - those described as pungent and decayed.

The band of scientists started with the foundation of Atlas of Odor Character Profiles, a 1985 book by Andrew Draveniek. After running 144 of these character profiles through the computer model, they found each of the smells could be lumped into ten categories.

According to scientist Jason Castro from Bates College, research into the mathematical and physical properties of smell isn’t easy, nor is it common.

“It’s an open question how many fundamental types of odor qualities there are,” said Castro in a statement. He added, "this is in striking contrast to olfaction's 'sister sense,' taste, where we know that five basic qualities seem to organize sensation."

With these smells now mathematically categorized, Castro and team say their research is still incomplete. Yet they already have an eye on how they could employ their results in a practical manner.

"We have not solved the problem of predicting a smell based on its chemical structure, but that's something we hope to do,” said Castro in an interview with the BBC.

“And obviously perfume companies, flavor and fragrance companies, are really interested in doing that well. This study supports the idea that the world of smells is tightly structured, and organized by a handful of basic categories.”

Before they can get to this point, however, Bates said they’ll need to put the computer model to the test and present it with more difficult scents to smell, analyze and categorize. Interestingly, though the research finds there are only ten general categories which humans can detect, Castro says natural smells can be built of different combinations of the smells, such as fragrant/resinous or citrus/woody.

In an interview with the BBC, a British smell expert said that this isn’t the first time a scientist has said humans can only smell a few specific groups of scents. This previous attempt to lump all smells into smaller groups was contested at the time, and professor Tim Jacob believes this could very well happen again.

"I'm sure that Castro et al's paper will 'stimulate a lot of useful thought’,” said Jacob in an interview.